The Cambrian Period records one of the most extraordinary transitions in the history of life. Beginning as simple sponges more than 635 million years ago, the earliest animals evolved into a diverse marine fauna over the course of 100 million years. In The Cambrian Explosion, Erwin and Valentine synthesize research from many fields to explain why there was such remarkable novelty of animal forms. This is an integrative work of the highest quality, covering one of the most fascinating and transformative periods in life's history.